Understanding Knee Osteoarthritis

The knee is the largest joint in the body and the one most susceptible to osteoarthritis. If you’re one of the millions of people who experience knee pain and stiffness caused by osteoarthritis, you may find it helpful to understand how this condition develops in the joint over time.

Read on to find out how the knee’s cartilage, bones, and other soft tissues gradually change during osteoarthritis. You can also see these changes in our Knee Osteoarthritis Video .

Knee osteoarthritis is a condition characterized by the breakdown of the cartilage and other tissues in the knee. This breakdown can lead to pain, swelling, and new or worsening inflammation.

The knee joint is located where the femur (thigh bone) meets the tibia (shin bone). The patella (kneecap) glides along a groove at the bottom of the femur. The fibula is a small long bone located next to the tibia and isn’t considered part of the knee joint.

See Knee Anatomy


Knee osteoarthritis has been traditionally thought of as the result of a previous injury (post-traumatic arthritis) or cumulative wear-and-tear on the joint. Research now suggests that low-grade inflammation throughout the body, joint biochemistry, and other factors also contribute to the development of osteoarthritis.1,2

See Knee Osteoarthritis Causes

Cartilage in the knee breaks down

The large, rounded areas of bone located at the top of the tibia and bottom of the femur are called condyles. The condyles move against one another when the knee bends. Two types of cartilage are located in this area:

  • Articular cartilage is a smooth, strong, slippery material that covers the ends of both condyles. Articular cartilage helps reduce friction as the knee bends and straightens.
  • The meniscus is a thick piece of cartilage layered in between the femur and tibia. The meniscus cartilage acts as the knee’s shock absorber.

There is also a layer of articular cartilage on the underside of the patella, or knee cap, to help it smoothly glide along the front of the femur.

A hallmark sign of knee osteoarthritis is damage to the knee’s articular cartilage. Meniscal cartilage is different from articular cartilage. While damage to the meniscus is considered a different medical condition than knee osteoarthritis, the two conditions are highly intertwined.3

See What Is Cartilage?

The knee’s bones undergo changes

As changes happen to the articular cartilage and soft tissues, so do changes to the joint’s bones. These changes may be visible on x-rays and other medical imaging. Three such changes are described below.

  1. Bone spurs (osteophytes). Growths called bone spurs may develop on the bone at the knee joint. While the bone spurs themselves are painless, they can cause more friction at the knee joint, potentially contributing to painful inflammation.
  2. Subchondral bone sclerosis. The surfaces of the tibia and femur that lie just beneath knee cartilage can change in composition and harden. This condition is called subchondral bone sclerosis.
  3. Bone lesions and cysts. The bone may develop cysts and areas of abnormal swelling called bone marrow lesions. These lesions may be associated with knee pain.4,5

The joint may undergo other changes, too. For example, a delicate membrane that surrounds the arthritic knee joint, called the synovial membrane, may become inflamed. This inflammation may result in knee pain, swelling, and stiffness.

See What Is a Synovial Joint?

Without treatment, joint degeneration gets worse

The first symptoms of knee osteoarthritis are often stiffness or aching pain in the knee. Without treatment to slow down the arthritic process, the condition is likely to get worse, and the thinning knee cartilage may begin to disappear completely.

See Knee Osteoarthritis Symptoms

There are many treatments that can help stop or slow down the progression of knee osteoarthritis. A health care practitioner can help you identify the best treatment options based on the severity of your arthritis, overall health, and lifestyle.

Read more about Knee Osteoarthritis Treatment


Arthritis doesn’t always cause symptoms

Keep in mind that knee pain is not always directly associated with the severity of arthritis.1 For example, a person’s x-rays may show mild knee joint degeneration, and they may feel a lot of pain or no symptoms at all.

Even at its later stages, osteoarthritis symptoms are unpredictable. It is possible to have "bone-on-bone osteoarthritis" and have mild or no symptoms. It is unclear why these differences exist.

Read more about osteoarthritis knee pain and how to treat it: What Is Knee Osteoarthritis?

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